Carbon black is an amorphous form of carbon which has varied uses, including use as a pigment and as a strengthening and reinforcing agent for many rubber products. Generally, it is commercially prepared by the partial combustion or thermal decomposition of hydrocarbons in the vapor phase.
Because of the amorphous nature of their carbon atoms, the physical properties of carbon blacks differ. The particular use of a carbon black is dependent upon these physical properties, more particularly, its chemical composition, pigment properties, surface area, state of subdivision, adsorption activity, colloidal properties, etc.
Due to the variable nature of the physical properties of carbon blacks, it is important that there is a high degree of product uniformity. One particularly useful manner of characterizing carbon black is by determining its iodine adsorption number. The iodine adsorption number is related to the surface area of carbon blacks and is generally in agreement with nitrogen surface area, although it is also affected by the presence of volatiles, surface porosity, and extractables.
During the commercial production of carbon black, it is necessary to monitor the carbon black being produced in order to ensure that the carbon black produced in the run falls within the desired specifications. This is often accomplished by taking samples and testing for the iodine adsorption number. The accuracy of the iodine number of a carbon black has become increasingly important in recent years as an aspect of quality control in the final product into which the carbon black is incorporated.
The iodine adsorption number of carbon black is commonly determined according to the industry-accepted standard volumetric test set forth in ASTM D1510.
According to this test, an adequate sample of carbon black is dried for one hour in an oven set at 125.degree. C. Thereafter, the carbon black is weighed to the nearest 0.000 lg. Next, the sample of carbon black is treated with 25 ml of 0.0473 N iodine solution (previously standardized). The mixture is capped and shaken, and then centrifuged. The supernatant solution is decanted, and 20 ml of the supernatant solution is pipetted into a flask. This solution is titrated with 0.0394 N sodium thiosulfate solution (previously standardized). When the solution turns pale yellow, five drops of starch solution are added and the titration is continued drop wise until the endpoint (colorless solution) is reached. The titration volume is recorded to the nearest 0.25 ml if done manually, or to the nearest 0.01 ml if done via a digital buret.
A blank iodine determination is made by pipetting 20 ml or dispensing 25 ml of 0.0473 N iodine solution into a flask and titrating with 0.0394 N sodium thiosulfate.
The iodine adsorption number is calculated to the nearest 0.1 g/kg according to the equation: EQU I=[(B-S)/B].times.(V/W).times.N.times.126.91
where
I=iodine adsorption number, g/kg,
B=ml of sodium thiosulfate required for the blank,
S=ml of sodium thiosulfate required for the sample,
V=calibrated volume of the 25 ml iodine pipet or dispenser,
W=grams of carbon black sample, and
N=normality of the iodine solution, and 126.91=equivalent mass of iodine.
The volumetric measurements of the aforementioned method and resultant calculation of the iodine number based on the same are limited with regard to precision. For example, the volumetric measurements used to standardize the sodium thiosulfate solution are limited by the accuracy of the human eye reading the volume in a glass buret, by the sensitivity of digital buret, and by the tolerance of the volumetric equipment.